Timber resources which provide whole wood for use in manufacturing wooden articles such as furniture and housing are becoming increasingly scarce with the passage of time. The use of composite materials constructed of wood elements has increased dramatically in recent years. Manufacturing of wood composite materials involves a degree of densification of the wood assembly as well as bonding wood elements together. Continuous presses have been used to densify and bond the wood elements together.
Two basic types of continuous press are described in the prior art. The first type uses a "caterpillar-type" conveyor chain as the means for transporting the wood element material through the press while simultaneously applying pressure. A major advantage of the conveyor chain is that it has a potentially high pressing capacity. However, it has reduced flexibility and cannot accommodate complex profiles. The second type utilizes an endless steel belt for conveyance and pressure application. The chief advantages of the endless steel belt system are flexibility and an ability to be bent over complex profiles. However, the endless steel belt system has restricted pressure application capability.
A common feature of existing continuous presses for the production of wood composites is the shape of the press profile. It typically consists of an infeed section of converging upper and lower endless belts or conveyor chains, followed by a straight section where the wood composite product is held at a constant dimension while being heated. The shapes and relative lengths of these two sections vary. An important criteria in specifying the length of the compacting section is the thickness of the wood composite being manufactured. A loosely layered wood element assembly at the infeed may typically be three times thicker than the finished pressed wood composite product. Therefore, while a short compressing infeed section may be sufficient for the production of thin panel products, a much longer compression infeed section is required in presses for manufacturing large profile structural wood composites. In a large profile press, the converging infeed section can be composed of linear converging surfaces or be formed of curved convex infeed surfaces. The significance of the actual shape of the press profile increases with increasing working depth.
Caterpillar-type conveyor chain designs are not extensively used in continuous press systems today because they have low flexibility. Straight rectangular links that form the conveyor chain can only be made efficiently to follow a straight path, if the links are at the same time designed to support the high pressures that are required in compressing large composite materials. In principle, if a straight rectangular plate (which the conveyor links in principle are) is forced to follow a curve, or is rotated, as is the case in a transition between two converging paths, the plate becomes supported on a single point or line. Tremendous forces are therefore encountered at localized areas. Thus, such situations are generally highly undesirable. Another serious deficiency of current conveyor chain-track systems stems from the low radius bending curvature that the material is forced to follow in a continuous transition between two different paths.
A further undesirable feature of a conveyor chain-track system is that overpenetration of the link into the product occurs as the link is being rotated through a transition stage as, for example, in transition from converging to parallel press section. The mechanics of the link's motion is such that it is forced to overcompress the wood assembly at its leading edge, and then to retract to follow the second trajectory of the press bed. Such over-penetration may be detrimental to the composite product. But mainly, it subjects the link to large forces at the time when it is insufficiently supported. For the above reasons, caterpillar type conveyor chain systems are presently restricted to low pressure applications, or are used in systems where only limited compacting is required.
Flexible steel belt systems overcome many of the disadvantages inherent in conveyor chain systems. Because of its flexibility, a thin steel belt can smoothly follow complex contours. However, a flexible steel belt system has drawbacks. It has low capacity to deliver power for compression of the wood composite. In addition, in order to transport the forces that the belt must deliver to compress the wood composite, the belt must carry stresses arising from traction tension created between the driving pulleys, and stresses developed by bending around the pulleys. An analysis of these stresses demonstrates that there is an optimum belt thickness and, therefore, a practical limit to the power that a steel belt system can transmit in a given situation. The amount of power required to manufacture a wood composite product is directly proportional to its cross-sectional size. Continuous steel belt systems inherently have sufficient capacity for production of thin panel products. But their capacity is insufficient for production of structural composite products of larger cross sections. This power deficiency is overcome in some existing systems by including in the system an additional pulling device, which is located after the press. This device can be a caterpillar conveyor chain type because no further compaction is involved. It is obvious that this approach involves a large degree of equipment duplication and thus is very costly. In addition, because pulling power is required to transport the composite assembly through the press, a full density product cannot be manufactured until some time after the assembly is sufficiently engaged in the pulling device. As a consequence, a large amount of reject material is produced at the beginning of a production run.
Because of the nature of the wood composite pressing process, the elements of the composite material, as it is being compressed, are subjected to bending. In a symmetrical press, the elements that are proximate to the press bed (that is, they are in the exterior regions of the assembly) are bent more than the elements in the interior of the assembly. As a consequence, the associated bending stresses will vary among the elements from the exterior to the interior. During the process of compaction, the elements are often fused into a single composite beam by forming pressures well before completion of the compacting process. This leads to the development of additional interior stresses in the composite. In the compacting stage, the composite beam has a wedge shape between any two cross sections, and therefore the outside wood elements near the press bed must span a longer distance than those in the centre of the beam. As the compaction process progresses, a strain gradient therefore develops throughout the cross-section, resulting in the development of compressive stresses in the wood fibre near the exterior of the composite beam and tensile stresses near the centre of the beam. At the end of the compaction cycle, the press created wedge shape of the beam is eliminated and the beam becomes uniform in its cross-section. In principle, the process of creating a straight cross-section from a wedge shape cross section is similar to force-bending a curved beam into a straight beam. The developed stresses will vary from tensile in the outer fibres to compressive in the centre of the beam.
Ensuing stresses in the composite product are a combination of the multiple stresses as described above. Their respective magnitudes depend on such factors as the depth of compaction, curvature of compaction, type and conditions of the wood components, and their interfaces. Eventhough the stress distribution throughout the cross-section of a composite beam is of a complex nature, some generalizations can nonetheless be made. Smaller compacting curvature will create stress distribution predominantly tensile along the outer fibres and compressive in the centre of the beam. As the compacting curvature is increased this dominance will diminish and the stresses due to the depth of compression will gain in significance. Conceivably, at a specific press curvature, the state of stress in the beam will be at minimum.
Apart from the foregoing, wood as a natural material exhibits significant rheological behaviour. Only a portion of the stresses developed during manufacturing will remain in the finished product as residual stresses. If the compression is asymmetrical, these stresses may cause a bow to form in the finished product. In the case of symmetrical compression, a problem may appear when the symmetry is upset by further processing. These considerations apply equally to caterpillar track systems and flexible steel belt systems.
Without exception, the prior art dealing with wood composite manufacture describes presses consisting of a linear compression span which is sometimes preceded by a converging compressing section.
U.S. Pat. Nos. 3,852,012, 3,851,685 and 4,283,246 disclose continuous presses using endless steel belts as the transporting and power transmitting means. They include compacting capability at the press infeed.
U.S. Pat. Nos. 3,111,149 and 4,468,188 describe presses that use endless steel belts, but they do not offer compacting capability.
The presses that are disclosed in these patents are capable of providing the required compacting contours, and thus are suitable for small sections, but they lack the power required to compact larger cross-section composites.
Other prior art describes continuous caterpillar and chain conveyor presses but these are useful only in low pressure applications where no large compaction is involved. U.S. Pat. Nos. 2,142,932; 2,027,657; 2,868,356; and 3,068,920 fall in this category. The press that is described in U.S. Pat. No. 3,120,862 is of the caterpillar chain type and suggests compaction at its infeed. But the specification is silent on the problem of supporting straight links in transition between two trajectories. This problem is also not recognized or acknowledged in U.S. Pat. No. 3,045,586, which teaches the use of a conveyor chain as a pressing means.
A method of including compaction in a caterpillar chain type continuous press can be found in U.S.S.R. patent No. 587,013. The caterpillar chain is equipped with rollers that in turn roll on stationary supports. As a method of press construction, this arrangement is undesirable because it limits maximum press pressures, it is maintenance intensive, and it demands special lubrication procedures. Further complications arise if press heating is included.
A somewhat similar design appears in U.S.S.R. patent No. 402,190, where an additional heating section is included, but it is remote from the caterpillar chain section. Compaction is achieved by using converging endless steel belts over an extended length. Construction of the caterpillar chain is similar to that described in U.S.S.R. patent No. 587,013. The rolling means is attached to the caterpillar chain. Compaction is achieved by the means of endless steel belts powered through friction by the caterpillar chain.
U.S. Pat. No. 4,517,148 is pertinent because it points out the significance of the size of the compressing radius in the case of a specific elongated lumber composite made of narrow strands and using a steel belt press. A radius of curvature of 30 to 50 feet is assumed to be sufficient for the production of such material. The system appears to be designed for use with a conventional press.
None of the prior art cited makes use of the rheological characteristics of wood to improve the pressing procedure of wood composites, or attempts to synchronize compression and stress relaxation, or uses a fully curved press profile, or addresses or solves the problem of transition and full support of a track segment travelling on two or more trajectories of different curvatures.